1. Field of the Invention
This invention relates generally to fiber optic continuity test systems and specifically to a single-ended tester capable of detecting discontinuities in an optical fiber with a high degree of resolution.
2. Description of Related Art
Fiber optic continuity test systems are usually either single ended or dual ended. Dual ended systems require access to both ends of the optical fiber to measure the amount of light transmitted through the optical fiber. However, in many applications, access to only one end of the fiber is possible. In such systems, single ended testers must be employed.
Many single ended testers utilize optical time domain reflectometry (OTDR). OTDR systems work by first transmitting pulses of light into a fiber and then measuring the light that is reflected back. The time that it takes for the reflected light to return corresponds to the distance it travels along the fiber. This allows the OTDR system to produce a fiber signature. Two types of reflections occur. Pulse reflections are generated at breaks or joints where the light pulse encounters something other than a continuous glass core. Back scatter reflections are generated uniformly along a fiber as the transmitted pulse travels through the fiber. The back scatter signal provides a measurement of fiber attenuation. OTDR systems are frequently used for finding breaks in communication cables which are typically several kilometers long. One-half meter is considered excellent resolution for an OTDR system.
In some systems, where only single ended testers can be used, one meter resolution is not acceptable. Laser initiated ordance systems are one example. In such a system, a break close to the fiber/ordnance interface could not be distinguished from the end of the optical fiber by an OTDR system. For example, a break only a millimeter from the fiber/ordinance interface would disable the laser ordinance system but would not be detected by an OTDR system. This is because an OTDR system would have to resolve spikes in a return signal only 67 picoseconds apart to distinguish two reflections originating one millimeter apart. Current OTDR systems cannot achieve this resolution.
A further discussion of fiber optic testing systems may be found in M. Bininstool, "Integrated OTDR/Throughout Loss Measurement System for Environmental Testing of Cabled Optical Fibers" in S.P.I.E., Volume 559, Fiber Optics: Short-haul and Long-haul Measurements and Applications II, (1985), and R. Dupuy, "The Present and Future OTDR" in S.P.I.E., Volume 559, Fiber Optics: Short-haul and Long-haul Measurements and Applications II, (1985).
Thus, it would be desirable to provide a single ended method of ascertaining fiber optic link integrity which can distinguish a break close to the fiber end.